A: The ultra-bright red LED has a forward voltage drop of about 1.9V so in theory with a 3V CR2032 cell you would want to use a current limit resistor something like a 68 ohm or a 100 ohm. However the coin cell can only deliver a limited amount of current so in reality you are often able to get away without using a current limit resistor. The colour changing LED is rated between 2.7V and 5V so would work fine off a coin cell battery, though it won?t be at full brightness again because the coin cell won?t deliver enough current to run at full brightness, but it will still work fine.